The invention relates to data communication systems, more particularly to all areas where radio receivers and channel estimators are used in digital systems, and even more particularly to methods and apparatuses that process a received radio signal that is characterized by an unwanted DC component.
Modern cellular systems, such as the GSM system, are based on digital communication. In order to cope with distortions, such as multi-path distortions, known symbol patterns, called training sequences, are sent at regular intervals. For instance in either of the GSM or future EDGE systems, which are Time Division Multiple Access (TDMA) systems, a training sequence is transmitted in each burst. The training sequence is used for synchronization, (i.e., to find the time position of the received sequence), and for estimation of the radio channel (i.e., to estimate the multi-path and fading characteristics of the radio channel). The radio channel and synchronization information are then used in the decoding process of the received signal. The training sequences are often chosen in such a way that the synchronization and channel estimation procedures are optimized for a radio receiver having some assumed ideal characteristics. For instance, in GSM or EDGE there are eight different predefined training sequences which are chosen such that the uncertainty in the synchronization position and channel estimation is minimized for the case in which the received signal is distorted only by inter-symbol interference (ISI) and white noise.
However, many practical receivers introduce other kinds of distortions. For instance, using a homodyne receiver, which is a very efficient receiver architecture from a cost, size and current consumption perspective, an unwanted DC offset is often introduced in the receiver that must be taken care of. The DC offset arises mainly from three different sources: (1) transistor mismatch in the signal path; (2) the Local Oscillator (LO) signal leaking and self-downconverting to DC through the mixer; and (3) a large near-channel interferer leaking into the LO and self-downconverting to DC (see, for example, E. Larson, xe2x80x9cRF and Microwave Circuit Design for Wireless Communicationsxe2x80x9d, Artech House Inc., Norwood, Mass., USA, 1996).
An exemplary technique for addressing such a DC offset is to extend the channel filter model with a DC tap. In FIG. 1A a conventional homodyne receiver with a DC offset and channel estimator and channel equalizer for use in a burst transmission system, such as the GSM or the EDGE systems, is shown. The received signal is first filtered by a bandpass filter (BPF) 10 designed to pass only the desired frequency band (for instance the GSM frequency band). The signal is then amplified in a low noise amplifier (LNA) 12. The output from the LNA 12 is a signal given as:
st=rf cos(xcfx89t+xcfx86t)
where xcfx89 is the carrier frequency, rt is the amplitude and xcfx86t is the phase information. This signal is down-converted to baseband In-phase (I) and Quadrature phase (Q) components in the mixers 20 and 30, respectively. The local oscillator (LO) 14 generates a local oscillator frequency adapted to the carrier frequency of the desired signal. The I and Q components are then low-pass filtered (LPF), by respective filters 22 and 32, and converted to a digital signal, by respective Analog to Digital (A/D) converters 24 and 34, and digitally low-pass filtered again, by respective filters 26 and 36, to obtain a signal format that can be handled by a Data Recovery (DR) unit 16 that demodulates the received signal.
Referring to FIG. 1B, the data recovery unit 16 works as follows. The complex valued baseband signal supplied to the DR unit 16 can be written as:                               y          t                =                                            I              t                        +                          jQ              t                        +                          I                              D                ⁢                                  xe2x80x83                                ⁢                C                                      +                          jQ                              D                ⁢                                  xe2x80x83                                ⁢                C                                              =                                                    ∑                                  i                  =                  0                                L                            ⁢                                                h                  t                                ⁢                                  u                                      t                    -                    i                                                                        +            m            +                                          e                t                            .                                                          (        1        )            
where hi are the channel filter taps, ut is the transmitted symbol, m is the unknown DC component generated in the radio receiver, and et is a white noise sequence (i.e., a sequence of independent random variables with zero mean). The baseband signal is supplied to a synchronization and channel estimation unit 50, which correlates the baseband signal (which is known to include a training sequence within the burst) with the known training sequence (TS) 60 in order to find the synchronization position. The synchronization and channel estimation unit 50 also estimates the DC offset and channel filter taps by means of known techniques, for instance by using a Least Squares (LS) algorithm. When using least squares, the channel estimate can be written according to:                               H          ^                =                                            (                                                ∑                                      k                    =                                          L                      +                      1                                                                            N                    TS                                                  ⁢                                                      U                    k                                    ⁢                                      U                    k                    T                                                              )                                      -              1                                ⁢                                    ∑                              k                =                                  L                  +                  1                                                            N                TS                                      ⁢                                          U                k                            ⁢                              y                                  k                  +                                      τ                    ⁢                                          xe2x80x83                                        ⁢                    sync                                                                                                          (        2        )            
where Ĥ=[ĥ0, . . . ,ĥL,{circumflex over (m)}]T, Uk=[uk, . . . ,uk-L,1]T, NTS is the number of training symbols, and xcfx84sync is the synchronization position. The estimated DC offset is then subtracted from the baseband signal and the resultant signal {haeck over (y)}t=ytxe2x88x92{circumflex over (m)} is fed to an equalizer 70 that uses the estimated channel filter taps to decode the signal.
Due to the need for estimating both the channel filter taps and DC offset in the presence of DC in the received signal, the optimized estimation characteristics for the training sequences are not realized, resulting in a different receiver performance for different training sequences used. The performance difference is due to the uncertainty (i.e., the variance) of the parameter estimate, which is proportional to the diagonal element of the matrix given as:   A  =                    (                              ∑                          k              =                              L                +                1                                                    N              TS                                ⁢                                    U              k                        ⁢                          U              k              T                                      )                    -        1              .  
From a theoretical point of view, in order to have good parameter estimates, one should choose the input vector Ut such that the matrix A is a diagonal matrix with identical or very close diagonal elements. (see, for example, L. Ljung, xe2x80x9cSystem Identificationxe2x80x94Theory for the Userxe2x80x9d, Prentice Hall Inc., New Jersey, 1987). The training sequences in EDGE and GSM assume that no DC component is present, and are chosen such that A is close to diagonal implying good estimation performance. However, a DC tap introduced in the U vector can cause the matrix to be far from a diagonal matrix, which results in a large performance loss due to bad channel and DC tap estimates. Therefore, a method that can enhance the synchronization and channel estimation procedure despite the existence of distortions such as a DC offset is needed.
It should be emphasized that the terms xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d, when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
The current invention overcomes the prior art limitations by providing a method and apparatus that offsets the frequency of a local oscillator in a radio receiver by an offset frequency wherein the offset frequency is based on a training sequence. The method and apparatus for generating a local oscillator frequency in a radio receiver comprises controlling the local oscillator such that it generates a signal having the local oscillator frequency that is equal to a carrier frequency of a received signal plus an offset frequency, wherein the offset frequency is based on a training sequence included in the received signal.